The present invention relates to a method of producing a photodiode and the photodiode produced with the method. More specifically, the present invention relates to a photodiode for generating an electrical current upon receiving light such as ultraviolet light, and a method of producing the photodiode.
Recently, an irradiation amount of ultraviolet light has increased due to destruction of the ozone layer, and ultraviolet light in sunlight has become a major concern on a human body and environment. In general, ultraviolet light is invisible light in an ultraviolet light region less than 400 nm, and is categorized into three types of light, i.e., long wave ultraviolet light (UV-A light, a wavelength between 320 nm and 400 nm); intermediate wave ultraviolet light (UV-B light, a wavelength between 280 nm and 320 nm); and short wave ultraviolet light (UV-C light, a wavelength of less than 280 nm).
The three types of light have different effects on a human body or environment depending on wavelengths thereof. More specifically, the UV-A light turns human skin to dark color, and reaches dermis to become a cause of aging. The UV-B light induces skin inflammation and skin cancer. The UV-C light has a strong germicidal action and tends to be absorbed by the ozone layer.
Since sunlight contains visible light and infrared light in addition to ultraviolet light, when a photodiode is used for detecting ultraviolet light, the photodiode needs to separate and detect only ultraviolet light in sunlight.
Patent Reference 1 has disclosed a conventional photodiode for detecting ultraviolet light. The conventional photodiode includes a semiconductor wafer having an SOI (Silicon on Insulator) structure, in which a silicon semiconductor layer with a thickness of about 150 nm is formed on a supporting substrate formed of silicon, and sandwiches an embedded oxide layer. An N-type impurity is diffused into the silicon semiconductor layer at a low concentration.
In the conventional photodiode, an N-type impurity is diffused into the silicon semiconductor layer at a high concentration to form an N+ diffusion layer having a comb shape. Further, a P-type impurity is diffused into the silicon semiconductor layer at a high concentration to form a P+ diffusion layer having another comb shape. The N+ diffusion layer and the P+ diffusion layer are arranged laterally such that the N+ diffusion layer and the P+ diffusion layer are nested with the silicon semiconductor layer in between. A metal wiring portion is electrically connected to the N+ diffusion layer and the P+ diffusion layer, so that a voltage is applied to the metal wiring portion for detecting ultraviolet light.    Patent Reference 1: Japanese Patent Publication No. 07-162024
In the conventional photodiode disclosed in Patent Reference 1, the silicon semiconductor layer with a thickness of about 150 nm is formed on the embedded oxide layer of the semiconductor wafer having the SOI structure. Accordingly, visible light passes through the photodiode, so that the photodiode detects only ultraviolet light. In the conventional photodiode, it is difficult to prevent an influence of reflection at an interface between the silicon semiconductor layer and the embedded oxide layer for the reasons described below, thereby making it difficult to accurately detect ultraviolet light in the ultraviolet light region less than 400 nm.
When visible light passes through the photodiode so that the photodiode absorbs only a wavelength range of ultraviolet, that is, selectively detects only ultraviolet light, a thickness of the photodiode can be determined as follows. First, an optical absorptance I/I0 of silicon can be expressed as an equation (1) according to Beer's law.I/I0=exp(−αZ)  (1)where α light absorption coefficient, Z is a light penetration depth, I is a light intensity at a depth Z, and I0 is a incident light intensity.
The light absorption coefficient α has wavelength dependence. According to the equation (1), a calculation is conducted for determining a wavelength at which the optical absorptance I/I0 becomes 10%. In the calculation, the optical absorptance I/I0 is determined at various thicknesses of a silicon semiconductor layer, and the wavelength at which the optical absorptance I/I0 becomes 10% is determined according to the thickness of the silicon semiconductor layer.
FIG. 8 is a graph showing a relationship between the thickness of the silicon semiconductor layer and the wavelength at which the optical absorptance I/I0 becomes 10%. As shown in FIG. 8, when the silicon semiconductor layer has a thickness of less than 50 nm, it is possible to selectively detect ultraviolet light in the ultraviolet light region less than 400 nm.
According to the calculation, an experiment was conducted for determining a sensitivity of a photodiode relative to light having various wavelengths. In the experiment, a semiconductor layer with a different thickness in a range less than 50 nm was formed on a semiconductor wafer having the Sal structure. Then, the photodiode of a lateral type was formed in the silicon semiconductor layer.
FIG. 9 is a graph showing the sensitivity of the photodiode and the wavelength of light. The silicon semiconductor layer of the photodiode had a thickness of 40.04 nm.
As shown in FIG. 9, when the photodiode includes the silicon semiconductor layer having a thickness of about 40 nm, a sub-peak exists at a wavelength region of visible light (violet light) longer than the ultraviolet light region less than 400 nm. Accordingly, the photodiode detects the wavelength region of visible light to generate a photo-electric current, and the photo-electric current is contained in a detected photo-electric current.
In the calculation described above, it is assumed that light passes through the silicon semiconductor layer. In an actual case, light is reflected at the interface between the silicon semiconductor layer and the embedded oxide layer. Accordingly, light passes through a shorter path and reacts with visible light having a wavelength shorter than the ultraviolet light region. As a result, the silicon semiconductor layer absorbs visible light, thereby causing the sub-peak.
When the silicon semiconductor layer has a smaller thickness, the sub-peak still appears. An experiment was conducted for determining a wavelength at which the sub-peak was observed in the silicon semiconductor layer having various thicknesses. FIG. 10 is a graph showing a relationship between a wavelength at which the sub-peak was observed and a thickness of the silicon semiconductor layer.
As shown in FIG. 10, when a thickness of the silicon semiconductor layer decreases, a wavelength at which the sub-peak was observed decreases. The relationship between the wavelength Ls (nm) at which the sub-peak was observed and the thickness Tsi (nm) of the silicon semiconductor layer cam be expressed as the following approximate equation (2).Ls=2.457×Tsi+312.5
From FIG. 10 and the equation (2), it is found that it is necessary to make the thickness of the silicon semiconductor layer less than 36 nm for preventing the adverse effect of light reflected at the interface between the silicon semiconductor layer and the embedded oxide layer, and for preventing the photodiode from reacting with visible light having a wavelength longer than 400 nm. That is, when the thickness of the silicon semiconductor layer is less than 36 nm, it is possible to prevent an error upon overlapping the photo-electric current in the visible light region with that in the ultraviolet light region due to the sub-peak in visible light region.
Based on the experiment described above, Patent Reference 2 has disclosed another conventional photodiode for detecting ultraviolet light. The conventional photodiode includes a first silicon semiconductor layer formed on an embedded oxide layer of a semiconductor wafer having the SOI structure. A first photosensitive element is formed in the first silicon semiconductor layer, in which a first P+ diffusion layer and a first N+ diffusion layer are arranged to face each other with a first P− diffusion layer in between.
Further, the conventional photodiode includes a second silicon semiconductor layer having a thickness smaller than that of the first silicon semiconductor layer. A second photosensitive element is formed in the second silicon semiconductor layer, in which a second P+ diffusion layer and a second N+ diffusion layer are arranged to face each other with a second P− diffusion layer in between. The first semiconductor layer has a thickness between 30 nm and 36 nm such as 35 nm, and the second semiconductor layer has a thickness between 3 nm and 30 nm such as 10 nm.    Patent Referent 2: Japanese Patent Application No. 2007-44465
In the conventional photodiode disclosed in Patent Reference 2, it is possible to accurately detect an intensity of ultraviolet light while preventing an influence of light reflected at an interface between the silicon semiconductor layer and an insulation layer. Further, it is possible to separate ultraviolet light in the three wavelength regions and detect intensities thereof through a calculation of two outputs of the first photosensitive element and the second photosensitive element.
In the conventional photodiode disclosed in Patent Reference 2, when the second silicon semiconductor layer is formed, after an element separation layer is formed with an LOCOS (Local Oxidation of Silicon) method, a sacrifice oxidation film is formed in a forming area of the second P− diffusion layer with a thermal oxidation method. Then, the sacrifice oxidation film is removed, thereby forming the second silicon semiconductor layer having a thickness smaller than that of the first silicon semiconductor layer.
After the second silicon semiconductor layer is formed, impurity ions are implanted into the first P− diffusion layer, the first P+ diffusion layer, the first N+ diffusion layer, the second P− diffusion layer, the second P+ diffusion layer, and the second N+ diffusion layer, respectively. Then, the impurity ions are diffused through a thermal processing, thereby forming the first photosensitive element and the second photosensitive element.
In the conventional photodiode disclosed in Patent Reference 2, the forming area of the second P− diffusion layer is processed to be in a thin film, thereby forming the second silicon semiconductor layer. Afterward, the impurity ions are implanted into each diffusion layer. Accordingly, when the impurity ions are implanted into the first silicon semiconductor layer with a large thickness at a high concentration to form the second P+ diffusion layer and the second N+ diffusion layer of the second photosensitive element, a rough surface tends to form in an upper surface of a portion of the second P− diffusion layer with a small thickness adjacent to the second P+ diffusion layer and the second N+ diffusion layer, thereby increasing a dark current.
In view of the problems described above, an object of the present invention is to provide a photodiode and a method of producing the photodiode capable of solving the problems of the conventional photodiode. In the present invention, the photodiode includes a photosensitive element having a low concentration diffusion layer with a thickness smaller than that of a P-type high concentration diffusion layer and an N-type high concentration diffusion layer. In the present invention, it is possible to reduce a dark current of the photodiode.
Further objects and advantages of the invention will be apparent from the following description of the invention.